How can I compare two serial port readings on Arduino Mega?

Question

I'm writing some code that compares information from two Arduino Unos by an Arduino Mega 2560. The problem that I'm having is that when I read the information coming in through Serial1 and Serial2 on my if statements, I get both inputs out on my serial monitor for the Mega, but if I try to compare them after I have read them it comes out not working the way it is intended. I don't know if I might be missing some syntax on my code or if I'm doing this wrong but I would appreciate any help I can get.

byte inData;
byte inDataTwo;
void setup() 
{
 // Open serial communications and wait for port to open:
 Serial.begin(9600);
 Serial1.begin(9600);
 Serial2.begin(9600);
 while (!Serial) 
 {
 ; // wait for serial port to connect. Needed for native USB port only
 }
}
void loop() 
{
 if (Serial1.available()>0 )
 {
 inData = Serial1.read();
 Serial.print(char(inData));
 }
 if (Serial2.available()>0)
 {
 inDataTwo = Serial2.read();
 Serial.print(char(inDataTwo));
 }
 if(inData == inDataTwo)
 {
 Serial.print("true");
 }
 else
 {
 Serial.print("False");
 }
}`

• varun: can you provide a sample output as well..!?
• Carl Fern: what do you mean sample output? when i get the information on the arduino mega i get the results from both my arduino unos and i can display them on the screen. im sending letters through a laser and the two arduinos convert the information back to characters and i can display that on the screen. when i connect them to the arduino mega the mega displays both of the same character on the screen. I know is working but when i compare them, the program seem to do not what i want to do
• Juraj: print them in else
• Vaibhav: can you tell what are you sending on serial terminal?
• Neal Rosenblum: Also, I see a small character after your last bracket for the loop(). I thought that it was a speck of dirt on my screen, but you actually have a character.

Answer 1

I think your code is implemented wrong in general. Even if it worked "like intended" it would not behave like you might want it to.

The first problem is, that you are polling for bytes that are received asynchronuously. That means that two byte sequences must not only be equal, they must also be sent "at the same time" if they should pass your comparison test. As the atmega will loop() very fast, there is only very little tolerance in timing.

Secondly, the variables you write received values to are not reset, meaning that once they have the same value they will pass the if statement until another byte has been received. Actually they aren't initialized and therefore will be 0 at the very beginning. So I wonder why you don't get your Serial interface flushed with true. Is this actually the entire code?

As a solution I suggest accumulating received bytes into a buffer and waiting for a soecific termination byte (often /n). If a termination character has been received, perform the comparison on that buffer.